While organic electroluminescent (EL) devices have been known for over two decades, their performance limitations have represented a barrier to many desirable applications. In simplest form, an organic EL device is comprised of an anode for hole injection, a cathode for electron injection, and an organic medium sandwiched between these electrodes to support charge recombination that yields emission of light. These devices are also commonly referred to as organic light-emitting diodes, or OLEDs. Representative of earlier organic EL devices are Gurnee et al. U.S. Pat. No. 3,172,862, issued Mar. 9, 1965; Gurnee U.S. Pat. No. 3,173,050, issued Mar. 9, 1965; Dresner, “Double Injection Electroluminescence in Anthracene”, RCA Review, 30, 322, (1969); and Dresner U.S. Pat. No. 3,710,167, issued Jan. 9, 1973. The organic layers in these devices, usually composed of a polycyclic aromatic hydrocarbon, were very thick (much greater than 1 μm). Consequently, operating voltages were very high, often greater than 100V.
More recent organic EL devices include an organic EL element consisting of extremely thin layers (e.g. <1.0 μm) between the anode and the cathode. Herein, the term “organic EL element” encompasses the layers between the anode and cathode. Reducing the thickness lowered the resistance of the organic layers and has enabled devices that operate at much lower voltage. In a basic two-layer EL device structure, described first in U.S. Pat. No. 4,356,429, one organic layer of the EL element adjacent to the anode is specifically chosen to transport holes, and therefore is referred to as the hole-transporting layer, and the other organic layer is specifically chosen to transport electrons and is referred to as the electron-transporting layer. Recombination of the injected holes and electrons within the organic EL element results in efficient electroluminescence.
Since these early inventions, further improvements in device materials have resulted in improved performance in attributes such as color, stability, luminance efficiency and manufacturability, e.g., as disclosed in U.S. Pat. Nos. 5,061,569, 5,409,783, 5,554,450, 5,593,788, 5,683,823, 5,908,581, 5,928,802, 6,020,078, and 6,208,077, amongst others.
U.S. Pat. No. 6,653,654 discloses OLEDs with group 10 metal complexes of bis-2,9-(2-hydroxyphenyl)phenanthrolines as emissive dopants.
U.S. Pat. No. 6,468,676 teaches the use of organic alkali metal complexes, including phenanthroline complexes, for an electron-injection layer located between the light emitting layer and the cathode.
U.S. Pat. No. 6,396,209 describes an OLED structure including a mixed layer of an electron-transporting organic compound and an organic metal complex compound containing at least one of alkali metal ion, alkali earth metal ion, or rare earth metal ion with a 2-(hydroxyphenyl)pyridine. Likewise, U.S. Pat. No. 6,703,180 and U.S. Pat. No. 7,198,859 also teach metal complexes of 2-(hydroxyphenyl)pyridines in a layer adjacent to an emitting layer on the cathode side. WO2004/081017 discloses OLEDs with aluminum complexes of 2-(hydroxyphenyl)pyridines in an electron transport layer.
US Patent Publication 2004/0207318 discloses OLED devices with an organic metal complex in an electron transport layer and a mixture of an electron transport material and an organic metal salt, including LiQ, in an electron injection layer.
Organometallic complexes, such as lithium quinolate (also known as lithium 8-hydroxyquinolate, lithium 8-quinolate, 8-quinolinolatolithium, or Liq) have been used in EL devices, for example see WO 0032717 and US Patent Publication 2005/0106412.
However, these devices do not have all desired EL characteristics in terms of high luminance in combination with low drive voltages. Thus, notwithstanding these developments, there remains a need to reduce drive voltage of OLED devices while maintaining good luminance.